Dear Changwon, i also own a Retina imac (i7 Radeon 295). Harddisk has been changed by the distributor – they sell customized "Edition iMacs". Now i have the same problem as you subscribed - from day one. Fan speeds up to 1700 / 1750 rpm after a few seconds after power on. Next week my Apple certified distributor will check my iMac and hopefully fix that "noisy" problem. I will inform you, when we will be successful ... Have a great time so long. Greetings, Markus (mr@i-map.de)